The present invention relates an energy absorber and a method for manufacturing the same. More particularly, the present invention pertains to a fiber-reinforced resin energy absorber that is located in a position to which impact is applied and to a method for manufacturing the energy absorber.
Energy absorbers are often provided in portions of a vehicle body that receive impacts, such as a front portion and a rear portion. An energy absorber is deformed when receiving impact, and is crushed to absorb energy. For example, a front side member and a rear side member of a vehicle each play a key role as an impact energy absorbing member (energy absorber). Using metal for energy absorbers increases the weight. Thus, to reduce the weight, energy absorbers are formed of fiber-reinforced resin.
Characteristics desired for such energy absorbers include the ability to be gradually crushed to stably absorb energy without significantly increasing the load required for crushing at an early stage of deformation. Among energy absorbers that have such characteristics, is there an energy absorber disclosed in U.S. Pat. No. 6,406,088. The thickness of this energy absorber is reduced toward the distal end in a direction along which compressive load is applied. FIG. 26 shows an energy absorber 61 disclosed in the publication. The energy absorber 61 is shaped like a rectangular tube. The thickness of a wall 61a is reduced toward the distal end and increases toward the proximal end (base). FIG. 27 shows a structure for varying the thickness of the wall 61a. In the structure shown in FIG. 27, reinforcing fibers of fiber-reinforced resin forming the energy absorber 61 are formed into layers of laminated fibers. In the layers, fiber bundles 62 extend in a direction along which a compressive load is applied to the energy absorber 61. The fiber bundles 62 have different lengths along the direction of compressive load.
The energy absorber 61 of the above patent publication is made of fiber-reinforced resin. The layers of the fiber bundles 62 having different lengths along the direction of compressive load applied to the energy absorber 61 are laminated. That is, the reinforcing fibers are formed of laminated fibers. This structure complicates the arrangement of the fibers. This is because, to laminate layers of fiber bundles 62 having different lengths, fiber bundles 62 that have been cut to predetermined variation of lengths must be prepared, and it is difficult to place each fiber bundle 62 while maintaining it in a linearly extending state.
FIG. 28 illustrates another energy absorber 41 of this type. The energy absorber 41 is cylindrical as shown in FIG. 28 and is made of fiber-reinforced resin. As reinforcing fibers, short fibers, long fibers, glass fibers, carbon fibers are used in combination as necessary (see Japanese Laid-Open Patent Publication 8-177922). A tapered portion 42 is formed at the distal end of the energy absorber 41. A θ fiber portion 43 is provided inside the energy absorber 41. A glass fiber portion 44 is provided about a distal portion of the θ fiber portion 43. A carbon fiber portion 45 is provided outside a proximal portion of the θ fiber portion 43. The θ fiber portion 43 has fibers arranged to be inclined by angle θ in positive and negative directions with respect to the axial direction of the cylinder. At a middle section of the θ fiber portion, the glass fiber portion 44 and the carbon fiber portion 45 are overlaid on each other. At an initial stage of a collision of the energy absorber 41, only the θ fiber portion 43 contributes to increase the crushing load. Also, because of the tapered portion 42, crushing starts at a relatively low load. Thereafter, the load required for crushing the section at which the glass fiber portion 44 and the carbon fiber portion 45 are overlaid on each other is increased, and the energy absorption amount is increased accordingly. As the crushing progresses further, the load required for crushing the carbon fiber portion 45 is further increased, which further increases the energy absorption amount.
In some types of fiber-reinforced resin that have fiber layers each having fiber bundles formed of filament fibers (continuous fibers), the fibers (fiber bundles) in each layer are arranged perpendicular to the fibers (fiber bundles) of other layers (arranged angles of the fibers are 0 degrees and 90 degrees). Such a fiber-reinforced resin has a higher strength compared to a fiber-reinforced resin having short fibers as reinforcing fibers. This type of fiber-reinforced resin (two-dimensional laminated fiber structure) is formed by laminating prepregs each having fiber bundles extending in a single direction, such that the directions of the fibers are different from one prepreg to another, and then hardening the resin.
When a force is applied to a two-dimensional laminated fiber structure along a direction perpendicular to its thickness, cracks are formed in a center portion along the thickness, which creates interlayer cracks. Therefore, if a two-dimensional laminated fiber structure is used to form an energy absorber, when the energy absorber is compressed, the property of resin between layers affects the energy absorption. This hampers the energy absorber from exerting the advantages of reinforcing fibers.
The energy absorber 41 shown in FIG. 28 uses various types of reinforcing fibers. That is, fiber materials are arranged such that the strength of the materials increases from an end at which crushing of the energy absorber 41 starts to the other end. Accordingly, a desired load-displacement variation is obtained. In this case, since compressive load required for crushing increases as crushing progresses along the axial direction of the energy absorber 41, the energy absorption amount can be increased compared to a case where reinforcing fibers of a single type are used. However, since a plurality of types fibers need to be prepared, the manufacture is troublesome. Further, no measures are taken against a rapid progress of cracks between adjacent fiber layers.